1. Field of the Invention
This invention relates to an analog-to-digital converter of the pulse width modulation type, and particularly to an incremental pulse width modulation type for generating a digital representation of the amplitude of an analog current with very low bias offset and bias drift errors.
2. Description of the Prior Art
This invention is an improvement over U.S. Pat. No. 3,918,050, entitled "ANALOG-TO-DIGITAL CONVERSION APPARATUS" and U.S. Pat. No. 4,112,428, entitled "CLOCKED PRECISION INTEGRATING ANALOG-TO-DIGITAL CONVERTER SYSTEM", both by A. K. Dorsman, and assigned to the same common assignee.
Prior Art systems, such as early three channel Analog-to-Digital Converter systems used in gyro stabilized cruise vehicle inertial guidance systems have been designed in which each channel had a dedicated precision current source and each current source had its own respective precision voltage reference. The drift characteristics of each respective channel in these early systems were influenced by the drift of the precision current source, the drift of each precision current source in turn being dependent on the drift characteristics of as many as five resistors in each current source circuit as well as the respective voltage source for the channel. The accuracy of these systems was also influenced by small changes in the input offset voltages, input bias currents and offset bias currents of error amplifiers or amplifiers used in integrator circuits.